V3 MicroSquirt® - QuickStart Guide

MicroSquirt® Dual Spark Appendix

Two-Cylinder, 4-Stroke Engines

Configuration 1 is for: For sequential ignition with 2 separate ignition coils and injectors. The dual spark mode is Falling/Rising Cam sync, with an M-N toothed crank wheel, N=0 or >0. Skip Teeth, which is the number of teeth between tach events, should be set to M (360 degrees for a crank wheel). Each Ignition output fires every 720° and they alternate. Fuel may be set so each injector fires every 720 degrees and they alternate, or they may be set Simultaneous, so both cylinders get injected at the same time every 360 degrees. Odd angle (output 2 ignition and fuel timing offset for odd-fire engines) may be 0 or non-zero.

Configuration 2 is for: Wasted spark mode with 2 separate ignition coils, injectors and a single crank synch from M-N wheel or with M-0 crank wheel and separate crank synch on Input 2. This mode is meant for odd-fire engines which can not use a single wasted spark coil because the required wasted spark timing does not coincide with the same crank position on both cylinders. So a total of 4 time-separated spark firings is required over the 720 degrees engine cycle.

Each Ignition output fires once every 360° and they are separated in angle by 180° + the Odd Angle offset. Likewise for the injectors. If Alternate injections is set, each injector squirts once per 360 deg, just like the ignition outputs; if Simultaneous fuel injection is set, both injectors would squirt at the same time, and this would make each cylinder get injected 4 times per 720° – which could be a problem for large injectors at low rpm or small injectors at high rpm.

Each Ignition output fires every 360 degrees and they alternate. Likewise for the injectors, which squirt immediately following the tach pulse, unless an Injector Start Delay as a percent of the tach interval is specified. If Alternate injections is set, only 1 injector squirts per tach pulse, just like the ignition outputs; if Simultaneous fuel injection is set, both injectors would squirt at the same time, and this would make each cylinder get injected 4 times per 720 degrees – which could be a problem for large injectors at low rpm or small injectors at high rpm.

Configuration 3 is for: Wasted spark mode with 1 ignition coil, 2 injectors, and with crank synch from M-N wheel or with M-0 wheel and separate crank synch input. This mode is meant for even-fire engines which can use a single wasted spark coil because the required wasted spark timing coincides with the same crank position on both cylinders. So 2 simultaneous spark firings will occur on each crank revolution.

Now we come to a problem. There is only one (dual wasted spark) coil, but 2 ignition outputs. What do we do with the extra output?

If we specify to the ECU that the 2 output FET drivers are tied together (done in TunerStudioMS and described in the initial main section), then each Ignition output will fire every 720° on alternate crank revolutions, so each cylinder gets spark every 360°, one of them being wasted.

The FET drivers are in a wired-or configuration and can be tied directly together. NEVER join the 2 processor outputs directly. If you don't have or want drivers, you can use IGN1 and IGN2 as inputs to an 'AND' or 'OR' logic chip or circuit (see this for circuit details).

The fuel injectors can be left untied, as in configurations 1 and 2, so, for Alternate setting, INJ1 injects cylinder 1 every 720 deg and similarly for INJ2, cylinder 2. Setting Simultaneous will cause each cylinder to be injected every 360 degrees. Either can be used since injector timing cannot be done properly in a wasted spark mode.

Leave the 2 ignition outputs separate and ignore IGN2 output: IGN1 output now fires the dual coil every 360 degrees, so cylinders 1 and 2 each get spark simultaneously every 360 degrees, but one is wasted.

The fuel injectors can again best be left untied, as in configurations 1 and 2, and, for Alternate setting, INJ1 injects cyl 1 every 360 deg and similarly for INJ2, cyl 2. Setting Simultaneous will cause each cylinder to be injected every 180 degrees, which is probably not desirable.

Which is better – configuration 3a or 3b? Here are the things that must be considered in making the tradeoffs:

Which method provides max dwell time at high rpm. This turns out to not be a significant factor because there really is no difference. This can be seen from the following timing diagrams, which show how each ignition channel operates over two tach cycles, but offset in phase:

So the dwell capability is the same, but note that there are twice as many vertical tics in the 3b IGN1 as there are for the 3a output. This is because the tach interval, is 180 degrees instead of 360 degrees. You have to generate twice as many sparks if you are not going to use the second channel. Since tach pulses trigger the processor to begin calculating spark and fuel for the next cylinder event, the processor is doing twice as many calculations as it needs to. While there is still headroom, those who want 20000+ rpm might want to consider tying the outputs together.

Configuration 3a provides a longer (double) interval between fuel injections. This makes more efficient use of the time for injecting at high rpm, since very little fuel is injected during topen, and if you have 2 topen times, that gives less total time for injecting. This extra time at high rpm would allow you to use smaller injectors, and so get better fuel control at idle. The choices are: 3a allows injecting each cylinder every 720 (360) degrees with alternating (simultaneous) injection mode; 3b allows injecting each cylinder every 360 (180) degrees with alternating (simultaneous) injection mode. Although it could be done, there is no sense in wiring injectors together – it is done far easier in the software by selecting Simultaneous mode. Also, if this is done, the x-tau calculation will be off, since the code will not know they are tied and will not calculate the correct time between cylinder squirts. To correct it would require tau to be halved when injector drivers are tied together.

Fuel requires more thought for the one cylinder because there is only one injector, but two fuel outputs. There are two possibilities:

If Alternating injections is selected, INJ1 will follow IGN1 and will inject cylinder 1 every 1440 deg. To get injections every 720 degrees you would have to tie the INJ1, INJ2 drivers together. This is not recommended nor necessary.

If Simultaneous injections is selected, INJ1 will inject cylinder 1 every 720 degrees and INJ2 can be left unconnected. This is the recommended option for sequential.

Each Ignition output fires every 720 degrees and they alternate. Since only one output is used, cylinder 1 again gets spark every 720 degrees.

If Alternating injections is selected, INJ1 1 will follow IGN1 and will inject cylinder 1 every 720 degrees. Thus INJ2 can be left unconnected. This is the recommended option for sequential in this case.

If Simultaneous injections is selected, INJ1 will inject cylinder 1 every 360 degrees and INJ2 should be left unconnected.

Configuration 5 is for: Wasted spark ignition mode with single crank synch from M-N wheel or with M-0 wheel and separate crank synch input. The ignition timing for the two possible configurations below looks identical to 3a and 3b above, including tach intervals of 360 degrees and 180 degrees.

Each Ignition output fires every 720 degrees and they alternate. Since they are tied together, cylinder 1 gets spark every 360 degrees, 1 of which is wasted.

If Alternating injections is selected, INJ1 1 will follow IGN1 and will inject cylinder 1 every 720 degrees. INJ2 can be left unconnected. If it is desired to obtain an injection every 360 degrees, you could connect the injection drivers, but this is not necessary nor recommended.

If Simultaneous injections is selected, INJ1 will inject cylinder 1 every 360 degrees and INJ2 can and should be left unconnected.

Each Ignition output fires every 360 degrees and they alternate. Since only one output is used, cylinder 1 gets spark every 360 degrees.

If Alternating injections is selected, INJ1 will follow IGN1 and will inject cylinder 1 every 360 degrees. Thus INJ2 can be left unconnected. This is the recommended option for wasted in this case.

If Simultaneous injections is selected, INJ1 will inject cylinder 1 every 180 degrees and INJ2 should be left unconnected. Unless multiple injections per engine cycle are needed, this option is not recommended.

Cam-Mounted Wheels

Configuration 6 is for: Use with 4-stroke engines equipped with a cam mounted M-N toothed wheel, so the mode is sequential. There are 3 sub-configurations here:

2 cylinders, 2 coils: Since this is a sequential mode, there is no reason to tie IGN1 and 2 together. Each generates one spark every 360 degrees of the cam wheel, and they are phase offset, which means each of the two cylinders is fired once per cam revolution, which is what is desired.

Using Alternating for fuel gives the same injection pattern as the ignition. Simultaneous would give 4 total injections per 360 cam degrees.

1 cylinder, 1 coil, sequential mode, with IGN1 and IGN2 tied together. Each then generates one spark every 720 degrees of the cam wheel, and they are phase offset, which means the connected IGN1 and IGN2 fire the single cylinder once per 360 degrees cam revolution, which is what is desired.

Using Alternating without tying INJ1 and INJ2 together would only give fuel to the single cylinder every 720 cam degrees. This is not recommended; instead, using Simultaneous means INJ1 and INJ2 fire simultaneously every 360 cam deg, so attaching only INJ1 to the single injector is all that is needed.

1 cylinder, 1 coil: Although this is a sequential mode, there is only one coil, so it is also allowed to use only IGN1 and ignore IGN2 output. With M / 2 Skip Teeth, IGN1 then generates one spark every 360 degrees of the cam wheel, which is what is desired.

Using Alternating and only using INJ1 (INJ2 is ignored) would give fuel to the single cylinder every 360 cam degrees, as desired. Simultaneous would give a squirt to the one cylinder every 180 cam degrees, which is generally not needed.

Two-Stroke Engines

Configuration 7 is for: 2-stroke, with either a cam or crank mounted M-N toothed wheel, so the mode is sequential. There are again 3 sub-configurations here, but selection of single cam or crank mounted wheel results in exactly the same behavior for each. In fact the 3 sub-configurations have exactly the same inputs and the output behavior is also exactly the same as for configurations 6a-6c above. The only difference that would be observed on the bench would be that if the wheel input was spinning at the same rpm, the rpm in configuration 6 would be double that in configurations 1-5 and 7. This is correct behavior and the reason is that the cam wheel for a 4-stroke turns half as fast as the crank, but for a 2-stroke, cam and crank turn at the same speed.

Note also, if rpm is half or double what it should be, something is wrong with your input configuration. Go over the documentation, make timing diagrams, use a scope or stimulator and try to understand why the original configuration did not give you what you wanted. Don't just change inputs until the rpm comes out right – this may not give you the optimum setup.